There are known magnetic encoders provided with a ring-shaped multipole magnet coaxially mounted on a rotating member of a measurement object, and Hall elements or other magnetic detection elements arranged so as to face the multipole magnetized surface formed on the external peripheral surface or the like of the multipole magnet. This type of magnetic encoder is disclosed in Patent Document 1.
The encoder device disclosed in Patent Document 1 is provided with two magnetic drums coaxially mounted on the rotating shaft of a motor or the like. A two-pole magnetization pattern is formed on the external peripheral surface of a first drum, and the external peripheral surface of the second drum is divided into 64 sections in the circumferential direction to form a 64-pole multipole magnetization pattern. First and second magnetic sensors are arranged at 90° angular intervals facing the magnetized surface of the first drum. Third and fourth magnetic sensors are arranged facing the magnetized surface of the second drum, and the angular interval of these magnetic sensors is set to an integral multiple of 90°.
With the encoder device of Patent Document 1, an A-phase signal and a B-phase signal that differ in phase by 90° are outputted from the first and second magnetic sensors when the rotating shaft rotates, and these output signals are converted to rectangular waves and inputted to a rotational direction discrimination circuit, whereby a +1 or −1 rotational frequency signal can be outputted for each rotation in accordance with the rotational direction. A C-phase signal and D-phase signal that differ in phase by integral multiples of 90° are outputted from the third and fourth sensors. In this encoder device, predetermined signal processing is carried out using computation means or a combination of circuits on the basis of the A- to D-phase signals, whereby the absolute rotational position can be detected with a resolution that corresponds to the number of poles in the multipole magnetization pattern.    [Patent Document 1] Japanese Laid-open Utility Model Application No. 06-10813